1. Field of the Invention
The present invention relates to a center take-off type power steering apparatus such as a hydraulic power steering apparatus, an electromotive power steering apparatus, and the like.
2. Description of the Related Art
Conventionally, in power steering apparats, a rack shaft is supported by a gear housing to be linearly movable, a power cylinder is coupled to the gear housing, the rack shaft is caused to pass through the power cylinder and propelling force generated by the power cylinder is imparted on the rack shaft, so that steering force can be assisted. In a center take-off type power steering apparatus of the power steering apparatuses which has been disclosed in Japanese Utility Model Application Laid-Open (JU-A) No. 57174275, a coupling member for coupling left and right tie-rods to an intermediate portion of the rack shaft is provided so as to be movable in a reciprocating manner within a window-shaped portion formed in a side wall of the power cylinder.
In the conventional art, however, there are the following drawbacks. (1) In the center takeoff type power steering apparatus, when positions of a gear housing and a power cylinder relative to each other regarding rotation direction about a common central axis thereof are even slightly deviated from each other at a connecting portion between the gear housing and the power cylinder, a coupling member between tie-rods and a rack shaft can not be moved smoothly due to the coupling member striking a window-shaped portion in an offset manner and the like so that the coupling member may be damaged. (2) A bellows-shape boot covering the window-shaped portion of the power cylinder comprises a singly body, one end of the boot being fastened to a mounting boss provided at the gear housing and the other end thereof being fastened to a mounting boss provided at the power cylinder. Bolts or the like for fixing tie-rod mounting members to the above-mentioned coupling member penetrate an intermediate portion of the boot. For this reason, holes of the boot which the above bolts or the like penetrate expand due to repetition of movement of the coupling member following leftward and rightward movements of the rack shaft, so that the boot is fatigued. As a result, the water-proof sealing property of the boot is damaged so that water or other foreign material may enter into the boot. (3) In the center take-off type power steering apparatus, the tie-rods are arranged in the vicinity of the boot, while the boot comprises an elastic body such as rubber. In order to prevent an intermediate portion of the boot positioned between both end portions thereof from deforming or hanging, it is desired to make the outer diameter of the boot large and make the inner diameter thereof small. In this case, however, the likely drawback is that the outer surface of the boot and the tie-rods interfere with each other, and the inner face of the boot and the rack shaft come in contact with each other.